It has already been proposed to apply strands carrying or impregnated with the above-mentioned adhesives on the surfaces of veneer sheets for the purpose of increasing the bonding strength of veneer sheets, in lieu of using a gummed tape an adhesive-backed tape.
With this method, it has been found that although an increased bonding strength between the veneer sheets is attained in the feeding direction of the veneer sheets due to the tensile strength of the strands applied thereon, the spliced veneer sheets will easily separate from each other due to a concentrated external force applied thereto when the spliced portions of the veneer sheets are subjected to a shearing or bending force.
This shortcoming is due to the fact that the surface area covered by strands bonded on the veneer sheets is so limited that it is difficult to maintain a sufficient bond between the veneer sheets. This has been the greatest disadvantage of the veneer sheet splicing apparatus of this kind.
In order to overcome this drawback, it has been attempted to apply a number of strands, conventionally, on one or both sides of the veneer sheets to maintain the bonding strength between each veneer sheet to be spliced.
This method, however, not only leads to economical disadvantages because an increased number of strands or quantity of adhesives must be used but, also, to the degradation of final products caused by the adverse effects of the thermoplastic adhesives being excessively mixed with the thermosetting adhesive which is used during the manufacturing process of plywood.
It is the object of present invention, therefore, to provide a method and an apparatus for splicing veneer sheets which provides an increased bonding strength between each veneer sheet in the transverse direction.
Essentially, according to the present invention, there is provided a method of splicing veneer sheets fed with appropriate spacings therebetween in a predetermined direction and then brought into substantially tight end-to-end abutment against each other, by cold pressing at least one strand supplied from above said fed veneer sheets and carrying with a thermoplastic adhesive onto the surface of a preceding veneer sheet by means of a cooling roll rotatably supported above said preceding veneer sheet at a first position, comprising the steps of: stopping the feed of a preceding veneer sheet at the moment the rear end of said preceding veneer sheet arrives at a second position on the infeed side of and spaced from said first position by a predetermined distance; feeding a succeeding veneer sheet until the front-end of said succeeding veneer sheet arrives at said second position; applying a thermoplastic adhesive bead in a molten state onto said strand at a localized portion thereof on the infeed side of the cooling roll, the length the strand between said first position and the localized portion of the strand is equal to the distance between said first and second positions, feeding a train of said preceding veneer sheet and said succeeding veneer sheet in said predetermined direction to make said thermoplastic adhesive bead pass said first position whereby said thermoplastic adhesive bead is cold pressed and applied onto the abutting end-portions of said preceding veneer sheet and said succeeding veneer sheet in the form of an adhesive film containing part of said strand therein, said adhesive film having a substantial area and spanning the preceding and succeeding veneer sheets.